Presently, study is being made by the 3GPP on a mobile communication system in which a radio access network of a UTRA (Universal Terrestrial Radio Access) scheme and a radio access network of an E-UTRA (Evolved Universal Terrestrial Radio Access) scheme are mixed.
In such mobile communication system, the mobile station UE is assumed to perform handover between a cell under the control of a radio base station of the UTRA scheme and a cell under the control of a radio base station of the E-UTRA scheme.